umass: Fix a direct call to sc_transform

Call umass_std_transform instead of following the sc_transform member to
get the standard transforms now that has moved to
umass_std_transform. Pass NULL as the CCB so that we don't stomp on the
status that was just set (and allow that in std_transform).

Sponsored by: Netflix
Differential Revision: https://reviews.freebsd.org/D49474